1. Field of the Invention
The present invention is directed to a circuit for combining the outputs of a plurality of Class-E RF power amplifiers, and is particularly directed to such a circuit which permits optimum operating conditions to be maintained when one or more of the power amplifiers is taken out of the circuit, such as by failure of that amplifier.
2. Description of the Prior Art
High power, solid-state RF systems customarily are created by summing the respective outputs of a plurality of RF power amplifiers modules, so as to deliver a single output signal. In systems wherein linear signal amplification is not required, it is desirable that the power amplifier modules operate in a highly efficient manner, and thus the power amplifiers in such systems are typically operated in Class C, D or E. For each of these classes of power amplifier operation, a unique and well-defined range of the power amplifier load impedances exists. Impedances outside of this defined range induce stresses in the amplified devices, which will result in the destruction of those devices.
In such systems, it is also usually desired that if one or more of the individual power amplifiers fails, the system can continue to operate, but at reduced power. In order to present a load to the remaining, operable power amplifiers which is still within the range of the acceptable values for reliable operation, means must be provided within the system to compensate for the effect which the failure of one or more of the power amplifier modules will have on the load impedance seen by the remaining, functional power amplifier modules.
Most combining systems currently in use are of the hybrid combiner type, in which any imbalance in the output voltage of the power amplifiers induces a voltage across a resistor, referred to as the "reject load." A hybrid combiner can be constructed so that regardless of the balance among the functioning power amplifiers, the load presented to those power amplifiers is a constant impedance. A significant disadvantage of known hybrid combining techniques is that, if one or more failed power amplifiers exists within the system, the power delivered to the load will be reduced by a factor proportional to the square of the number of failed power amplifiers, even though the power generated by each of the remaining power amplifiers is unchanged. Therefore, if one-half of the power amplifiers within a system using a hybrid combiner have failed, the total system output is halved as well. In this case, however, this power is divided equally between the reject load and the output load. Consequently, the system output is one-fourth of the original power. Additionally, the reject load must be capable of dissipating up to one-quarter of the system power. In a high power system, the reject load can therefore be an appreciable portion of the system cost.
Other combining circuits are known which dispense entirely with the use of a reject load, however, in such systems, for certain power amplifier failure modes, an excessive VSWR will be presented to the remaining power amplifiers, and the system must be shut down to protect those remaining power amplifiers.
A Class-E power amplifier, as exemplified in U.S. Pat. No. 3,919,656 (Sokal et al.) has high operating efficiencies due to the elimination of switching losses. Consequently, Class-E power amplifiers are used in numerous RF power systems. A known, representative Class-E circuit is shown in FIG. 1. The Class-E power amplifier therein is an RF power device, shown in FIG. 1 as a power transistor Q, which is operated as a switch so as to be driven "on" during 180.degree. of the RF cycle. Direct-current power V.sub.CC is delivered from a source to the switch via an RF choke L1. The switch formed by the transistor Q is shunted by a capacitive reactance, represented by C1, which absorbs any parasitic capacitance present within the switching device. The load network is shown in FIG. 1 as the series combination of C2, L2 and R1. This load network, in combination with the capacitance C1, is tuned for optimal load impedance. The nature of the unique optimal power amplifier load impedance of the Class-E power amplifier circuit permits its application in power combining circuits which have numerous advantage over other types of known circuits. It remains a problem, however, to provide a circuit for combining multiple Class-E RF power amplifiers in such a manner as to allow operation of this system in the presence of failed power amplifiers, with a lesser reduction in the system output power than occurs in a convention hybrid combiner, while utilizing only reactive circuit components, and thus eliminating the requirement for a reject load.